Handle ExternalStrings directly in the serializer without ObjectVisitor.

src/objects.h

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_OBJECTS_H_
 #define V8_OBJECTS_H_
 
 #include <iosfwd>
 #include <memory>
 
@@ skipping 10180 matching lines @@
 
   // Visits a code entry in a JS function.
   virtual void VisitCodeEntry(Address entry_address);
 
   // Visits a global property cell reference in the instruction stream.
   virtual void VisitCell(RelocInfo* rinfo);
 
   // Visits a runtime entry in the instruction stream.
   virtual void VisitRuntimeEntry(RelocInfo* rinfo) {}
 
-  // Visits the resource of an one-byte or two-byte string.
-  virtual void VisitExternalOneByteString(
-      v8::String::ExternalOneByteStringResource** resource) {}
-  virtual void VisitExternalTwoByteString(
-      v8::String::ExternalStringResource** resource) {}
-
   // Visits a debug call target in the instruction stream.
   virtual void VisitDebugTarget(RelocInfo* rinfo);
 
   // Visits the byte sequence in a function's prologue that contains information
   // about the code's age.
   virtual void VisitCodeAgeSequence(RelocInfo* rinfo);
 
   // Visit pointer embedded into a code object.
   virtual void VisitEmbeddedPointer(RelocInfo* rinfo);
 
@@ skipping 33 matching lines @@
   }
 };
 
 
 }  // NOLINT, false-positive due to second-order macros.
 }  // NOLINT, false-positive due to second-order macros.
 
 #include "src/objects/object-macros-undef.h"
 
 #endif  // V8_OBJECTS_H_